1. Field of the Invention
The present invention relates to joining ends of elastomeric sheet material having parallel extending reinforcing members. Particularly, the present invention relates to joining ends of member reinforced elastomeric sheet material used in the construction of a tire.
2. Description of the Prior Art
A typical radial tire includes a carcass ply which is made from a strip of elastomeric sheet material having a plurality of parallel extending reinforcing members. The carcass ply is formed into a generally toroidal shape. Each reinforcing member in the carcass ply is contained in a plane extending substantially radially of the axis of rotation of the tire. The ends of the strip of elastomeric sheet material of the carcass ply are overlapped to form a joint and provide a relatively airtight article. The joint area of the carcass ply has an arcuate length taken in a direction perpendicular to the extent of reinforcing members in the range of approximately one-quarter inch to one inch.
The joint area of the carcass ply has a density of reinforcing members which is approximately double the density of reinforcing members in any other area of the carcass ply. This results in a joint area which has a tensile strength resisting tension in the reinforcing members due to tire inflation which is approximately twice the tensile strength of any other area of the carcass ply. The double strength of the joint area may result in a visable sidewall indentation in the tire. Furthermore, when the tire is tested for uniformity, a variation from the applied test load is often attributable to the double strength of the joint area.
U.S. Pat. No. 4,466,473 discloses one end of a strip of fiber reinforced elastomeric sheet material comprising the tire carcass ply being cut at several locations. The cut extends in a direction perpendicular to and across the reinforcing fibers for a distance approximately equal to the overlap distance. The cut end of the strip contributes no tensile strength to the joint for resisting tension in the reinforcing fibers due to inflation. Thus, the joint has a tensile strength which is substantially equal to the tensile strength of any other area of the tire carcass ply.
U.S. Pat. No. 4,261,393 discloses a carcass ply for a radial passenger tire including two or three strips of elastomeric sheet material cut and joined together at end portions. The end portions of the strips of elastomeric sheet material are specially made for the purpose of forming a joint.
In one embodiment of U.S. Pat. No. 4,261,393, the end portions of each strip of elastomeric sheet material used to form a joint contain reinforcing fibers of a smaller diameter than fibers in a portion of the strip intermediate the end portions. The smaller diameter reinforcing fibers have a proportionately lower tensile strength than the reinforcing fibers in the intermediate portion of the strip. When the end portions are overlapped and vulcanized, the smaller diameter reinforcing fibers form a joint which contains more fibers but has substantially the same tensile strength as the intermediate portion of the strip.
In another embodiment of U.S. Pat. No. 4,261,393, the end portions of each strip of elastomeric sheet material contain reinforcing fibers of a diameter equal to the reinforcing fibers in the intermediate portion of the strip. However, adjacent reinforcing fibers in the end portions of the strip are spaced apart a greater distance than adjacent reinforcing fibers in the intermediate portion of the strip. When the end portions are overlapped and vulcanized, reinforcing fibers from one of the end portions are forced between reinforcing fibers of the other end portion. The resulting spacing between adjacent reinforcing fibers is approximately the same as between adjacent reinforcing fibers in the intermediate portion. Thus, the tensile strength in the joint area is substantially equal to the tensile strength in the intermediate portion of the strip of elastomeric sheet material.